Earnings per share (Details) - CAD ($) $ / shares in Units, $ in Thousands	12 Months Ended
Sep. 30, 2025	Sep. 30, 2024
Earnings per share [line items]
Basic net earnings	$ 1,658,285	$ 1,692,715
Diluted net earnings	$ 1,658,285	$ 1,692,715
Basic weighted average number of ordinary shares outstanding (in shares)	222,693,319	228,074,108
Dilutive net effect of all instruments on weighted average number of ordinary shares (in shares)	2,796,025	3,598,753
Diluted weighted average number of ordinary shares outstanding (in shares)	225,489,344	231,672,861
Basic earnings per share (CAD per share)	$ 7.45	$ 7.42
Diluted earnings per share (CAD per share)	$ 7.35	$ 7.31
Class A subordinate voting shares \| Capital stock
Earnings per share [line items]
Anti-dilutive securities excluded from computation of earnings per share (in shares)	2,247,354	6,528,608
PSUs \| Class A subordinate voting shares \| Capital stock
Earnings per share [line items]
Anti-dilutive securities excluded from computation of earnings per share (in shares)	8,861,543	2,601,356

Antidilutive Securities Excluded From Computation Of Earnings Per Share, Amount 1

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The weighted average number of ordinary shares outstanding plus the weighted average number of ordinary shares that would be issued on the conversion of all the dilutive potential ordinary shares into ordinary shares. [Refer: Ordinary shares [member]; Weighted average [member]]

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The amount of profit (loss) attributable to ordinary equity holders of the parent entity (the numerator) divided by the weighted average number of ordinary shares outstanding during the period (the denominator).

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The amount of profit (loss) attributable to ordinary equity holders of the parent entity (the numerator), divided by the weighted average number of ordinary shares outstanding during the period (the denominator), both adjusted for the effects of all dilutive potential ordinary shares. [Refer: Ordinary shares [member]; Weighted average [member]]

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The potential dilutive effect on the weighted average number of ordinary shares that relates to the assumed conversion of the entity’s instruments. It represents the aggregate of the reconciliation amounts for all classes of instruments that reflect dilutive effect on weighted average number of ordinary shares.

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Line items represent concepts included in a table. These concepts are used to disclose reportable information associated with members defined in one or many axes of the table.

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The profit (loss) attributable to ordinary equity holders of the parent entity. [Refer: Profit (loss)]

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The profit (loss) attributable to ordinary equity holders of the parent entity, adjusted for the effects of all dilutive potential ordinary shares. [Refer: Profit (loss)]

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The number of ordinary shares outstanding at the beginning of the period, adjusted by the number of ordinary shares bought back or issued during the period multiplied by a time-weighting factor.

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